1. Technical Field
This invention relates generally to input and output scanners, and more particularly to a scanner with a linearized pixel clock that compensates for scanner non-linearity.
2. Background Information
A scanner includes some type of scanning means for directing a light beam to a spot on a surface to be scanned. It does so in such a way that the spot moves across the surface along a scan line in a precisely controlled manner. That enables various input and output functions such as reading a document or printing a page.
Scanner non-linearity refers to variations in spot velocity occurring as the spot moves along the scan line. It is typically caused in such systems as polygon or galvanometer laser scanner systems by system geometry or a velocity variation of the scanning means and it can affect scanner performance. A scanner having a multifaceted rotating polygon, for example, directs the light beam at a constant angular velocity. But the spot is farther from the polygon facets at the ends of the scan line than it is at the center and so spot velocity increases as the spot moves from the center toward the ends. That can result in uneven pixel spacing along the scan line, a condition sometimes called pixel position distortion.
Some scanners compensate for scanner non-linearity electronically in order to reduce pixel position distortion using a linearized pixel clock. The pixel clock produces a pulse train that is used to turn the light beam on and off at each desired pixel position along the scan line, and it is said to be linearized in the sense that timing circuitry varies pulse timing according to spot position along the scan line and thereby according to spot velocity. That is done to at least partially compensate for scanner non-linearity in order to reduce pixel position distortion and produce more evenly spaced pixels.
Consider, for example, a scanner having a nine-inch scan line and a resolution of 300 dots-per-inch (dpi). That means there are 2700 pixel positions along the scan line. Ideally, the center-to-center spacing between any two adjacent pixel positions would be 1/300 inch so that they are evenly spaced. To accomplish that, each pulse in the pulse train must occur at just the right time to compensate for varying spot velocity. In other words, the time interval between each pulse and the following pulse must bear some defined relationship to spot position along the scan line and thereby spot velocity.
But it is difficult to produce such a pulse train. U.S. Pat. No. 4,729,617, for example, describes a scanning clock generating device having a voltage controlled oscillator (VCO). Timing circuitry varies its frequency according to spot velocity using variable frequency division of a fixed oscillator to produce reference pulses that control the VCO. Somewhat complicated logic and frequency dependent componentry are involved, however. So it is desirable to have some other way of providing a linearized pixel clock.
Another problem concerns the variations in pixel exposure resulting from the variations in spot velocity. Sometimes referred to as pixel exposure distortion, it can result in objectionable variations in shade despite compensation for scanner non-linearity that reduces pixel position distortion. Although it is conceivable to vary the intensity of the light beam according to spot position along the scan line offset that effect, accurate intensity control may be difficult and expensive to achieve. Thus, it is desirable to have some other way to reduce unwanted variations in shade of the type described.
Still another problem concerns the variation in pixel "blur," the result of pixel motion during the exposure, which causes the size of the pixel exposure to be increased somewhat over the size of the scanning spot. In other words, the spot is moving along the scan line during exposure. In a typical, scanner, the spot size is designed to be somewhat smaller than the pixel size in the scan direction such that with the addition of exposure blur the pixel exposure will be the correct size. The effect of spot blur is that the resolution capabilities of the scanner are somewhat less than the resolution capabilities of the optical system. Also, with a constant exposure time for each pixel, the variation of spot velocity will cause a variation in the degree of spot blur, thus affecting the size of the pixel exposure. In scanner systems that have a large variation in spot velocity, this variation of pixel exposure size may be significant. Thus, it is desirable to have some way to reduce or eliminate such pixel blur and to eliminate or reduce the variation of pixel exposure size.